The known photo-cathodes for emitting an electron (photoelectron or primary electron) corresponding to an incident photon include so-called transmission type photo-cathodes formed on a transparent substrate, and so-called reflection type photo-cathodes formed on a metal substrate such as Ni, and such photo-cathodes are adopted as important components, for example, in electron tubes such as photomultiplier tubes, photo-tubes, image intensifiers, streak tubes, and so on.
Many of the photo-cathodes now in practical use are made from a photoelectron emitting material containing an alkali metal (primarily, an intermetallic compound or a compound semiconductor), e.g., an intermetallic compound of Sb and Cs.
Conventionally, the photoelectron emitting material containing the above alkali metal as a constituent element is formed by generating the vapor of the alkali metal in an ambience held at a predetermined vacuum (preferably, 10−7-10−2 Pa in terms of partial pressure of residual gas) and temperature and reacting the alkali metal vapor with a constituent material of the photoelectron emitting material that is to react with the alkali metal. In an example of forming the photoelectron emitting material of the intermetallic compound of Sb and Cs, for example, a deposited film of Sb, which is a constituent material of the photoelectron emitting material to react with the alkali metal, is first formed on a substrate, and the vapor of Cs is then generated to react Cs with the deposited film of Sb, thereby forming a layer of the intermetallic compound.
In this case, the alkali metal is extremely instable in the atmosphere and therefore the alkali metal itself cannot be used as a source of the vapor of the alkali metal. It is thus common practice to use a supply source (so called an alkali source or alkali metal source) containing as a constituent a combination of an oxidizer with a reducer capable of generating the alkali metal by oxidation-reduction (redox) reaction at predetermined temperature. Examples of this supply source used heretofore include powder alkali metal sources, and alkali metal sources pressure-formed (pressed) in a pellet form. In the present specification, the alkali metal source (supply source) for the alkali metal vapor containing the aforementioned oxidizer and reducer will be referred to as an alkali metal generating agent.
These powder alkali metal generating agents or pelletized alkali metal generating agents are normally used in a state in which the generating agent is housed in a metal case provided with an aperture enough to discharge the alkali metal vapor to the outside. Furthermore, this metal case is also used as enclosed in a glass ampule in certain cases. Then this metal case is heated in formation of the photo-cathode to generate the alkali metal vapor.
Furthermore, the alkali metal generating agent is also used, for example, in formation of the secondary-electron emitting surface of dynodes in photomultiplier tubes.
An example of such alkali metal generating agents used conventionally is a powdered or pelletized alkali metal generating agent containing Si, Ti, Al, or the like as a reducer and containing as an oxidizer a chromate with an alkali metal ion as a counter cation (e.g., Cs2CrO4 or the like). The alkali metal generating agent containing this oxidizer is disclosed, for example, in Japanese Patent Applications Laid-Open No. 55-78438 and Laid-Open No. 53-124059.